Downlink solution for seamless and lossless cell change in a cellular system

ABSTRACT

The present invention relates to a UE, base station network node and methods thereof in a cellular mobile communication system. One method concerns handover from a first radio base station to a second radio base station wherein, it comprises the steps receiving a notification from the first radio base station that currently serves said UE, said notification indicating the last packet to be transmitted to this user equipment from said first radio base station on a first connection; and informing the second radio base station by a request to commence transmission from this second radio base station when the notified last packet is received at the UE.

FIELD OF THE INVENTION

The present invention relates to methods and arrangements in cellularmobile communication systems, in particular to seamless and losslesscell changes in the downlink of such communication systems.

BACKGROUND OF THE INVENTION

The present invention relates to methods and arrangements in a userequipment (UE) and in a radio access network of a cellular mobilenetwork. An example of such a radio access network is the UMTSterrestrial radio access network (UTRAN). The UTRAN is illustrated inFIG. 1 and comprises at least one Radio Network System 100 connected tothe Core Network (CN) 200. The CN is connectable to other networks suchas the Internet, other mobile networks e.g. GSM systems and fixedtelephony networks. The RNS 100 comprises at least one Radio NetworkController 110. Furthermore, the respective RNC 110 controls a pluralityof Node-Bs 120,130 also referred to as radio base stations. The Node Bsare connected to the RNC by means of the Iub interface 140. Each Node Bcovers one or more cells and is arranged to serve the User Equipment(UE) 300 within said cell. Finally, the UE 300, also referred to asmobile terminal, is connected to one or more Node Bs over the WidebandCode Division Multiple Access (WCDMA) based radio interface 150. Thenetwork of FIG. 1 is also referred to as a WCDMA network and is based onthe WCDMA standard specified by the 3:rd Generation Partnership Project(3GPP).

For cellular networks that support full mobility, it is a particularchallenge to support fast, seamless and lossless cell changes. This istrue both for already deployed systems, e.g. GSM/GPRS, WCDMA, CDMA2000,and also for future systems, e.g. those as being referred to as 3GPPUTRAN Long Term Evolution (LTE) or 4G-systems. “Seamless” means herethat the transmission is continuous, i.e. that there is no break in thetransmission during the handover (cell change). “Lossless” means that nopackets are lost during the handover. A further challenge is to avoidtransmission of packet duplicates caused by the handover.

Requirements for mobile data access are increasing and demand forbandwidth is growing. To meet these needs the High Speed Data PacketAccess (HSDPA) specification has been defined. HSDPA is based on WCDMAevolution standardized as part of 3GPP Release 5 WCDMA specifications.HSDPA is a packet-based data service in WCDMA downlink with datatransmission peak rate up to 14.4 Mbps over a 5 MHz bandwidth. ThusHSDPA improves system capacity and increases user data rates in thedownlink direction. The improved performance is based on adaptivemodulation and coding, a fast scheduling function and fastretransmissions with soft combining and incremental redundancy. HSDPAutilizes a transport channel named the High Speed Downlink SharedChannel (HS-DSCH) that makes efficient use of valuable radio frequencyresources and takes bursty packet data into account. This is a sharedtransport channel which means that resources, such as channelizationcodes, transmission power and infra structure hardware, is sharedbetween several users.

With HS-DSCH, there is a new HARQ retransmission layer defined in theNode B. HARQ is a fast and resource-efficient method for combatingtransmission errors. However, this new HARQ layer means that bufferingtakes place in the radio base station, making thus seamless and losslesshandovers a particular challenge. It is expected that the outcome fromthe 3GPP UTRAN LTE work will also include a realization with a fast HARQand a scheduler residing in the radio base station.

In short, a typical HS-DSCH handover procedure is now explained. Attimes of handover, the controlling node, i.e. the RNC in a WCDMA system,assigns a certain time-offset (activation time), so that the involvedradio base stations, i.e. the Node Bs in a WCDMA system, and the userequipments (UE) can prepare for the handover. These preparations includereceiving necessary control information, but also the transmission ofany remaining data in the buffer of the “old” radio base station, i.e.the radio base station that the UE is connected to before the handover.When the time-offset has elapsed, the handover is executed. After thehandover, the scheduler in the “new” radio base station, i.e. the radiobase station that the UE is connected to after the handover isperformed, is responsible for assigning grants and scheduling packets.Any data remaining in the “old” radio base station is discarded andpossibly recovered by some outer-layer ARQ, e.g. RLC terminated in theRNC. The HS-DSCH handover is further described in the specification TS25.931 issued by the 3GPP.

A drawback with the above described handover procedure, is that it isvery difficult to assign a suitable time-offset (activation time). Asuitable time-offset is required in order to be able to successfullytransmit all the packets that are forwarded via the “old” radio basestation, e.g. a Node B1. Furthermore, it is also desired that thetransmission is seamlessly continued from the “new” radio base station,e.g. Node B2.

Turning now to FIG. 2 illustrating the problem with handover activationtime. FIG. 2 illustrates a UE that is connected to Node B1 and is aboutto perform a handover to Node B2. Each Node B comprises a buffer forbuffering incoming packets from the controlling node RNC. Packets #1 to#5 have been forwarded to Node B1, which is currently responsible fortransmitting to the UE. However, the RNC does not have preciseinformation whether the packets have been delivered to the UE or not. Inthe present example it is assumed that packets #3, #4, and #5 stillreside in the Node B1 buffer (or are delayed in the transport network)and queuing for transmission to the UE.

For the example shown in FIG. 2, when supposing that the RNC nowexecutes a handover to Node B2 with a certain time-offset, packet #6 andonwards are then routed to Node B2 awaiting the handover to be properlyexecuted. However, if the offset is too low, some (or parts of) packets#3, #4, and #5 may not get enough time for transmission, and will bediscarded resulting in losses and if the offset is too high, packets #3,#4, and #5 may be transmitted well on time but there may then be adiscontinuity in the transmission, as packet #6 cannot be transmittedbefore the execution of the handover.

SUMMARY OF THE INVENTION

As stated above, to estimate a suitable time-offset can be verydifficult as the transmission time for the packets residing in the NodeB1 buffer is strongly dependent on factors such as the link quality tothis user and the load on the shared channel. Therefore with existingmethods, it is very difficult to achieve fast, seamless and losslesshandovers at the same time.

Thus, the object of the present invention is to achieve both losslessand continuous transmission at a downlink handover.

The object of the present invention is achieved by the methods andarrangements defined by the independent claims. Preferred embodimentsare defined by the dependent claims.

The network node according to the present invention comprising means fordetermining the last packet to be transmitted form the first radio basestation on a first connection and means for informing the UE of the lastpacket to be transmitted to the UE from said first radio base station onsaid first connection, makes it possible to achieve both lossless andcontinuous transmission at a downlink handover.

The first base station according to the present invention comprisingmeans for receiving information of the last packet to be transmitted tothe UE from said first radio base station (Node B1) on a firstconnection, and means for informing the UE of the last packet to betransmitted to the UE from said first radio base station (Node B1) onthe first connection, makes it possible to achieve both lossless andcontinuous transmission at a downlink handover.

Further, the UE according to the present invention comprising means forreceiving a notification from the first radio base station (Node B1)that currently serves said UE, said notification indicating the lastpacket to be transmitted to this user equipment from said first radiobase station (Node B1) on a first connection;

and means for informing the second radio base station (Node B2) by arequest to commence transmission from this second radio base station(Node B2) when the notified last packet is received at the UE, makes itpossible to achieve both lossless and continuous transmission at adownlink handover.

The invention also relates to methods. The method in a network nodecomprising the steps of determining the last packet to be transmittedfrom the first radio base station on a first connection, and informingthe UE of the last packet to be transmitted to the UE from said firstradio base station (Node B1) on said first connection, makes it possibleto achieve both lossless and continuous transmission at a downlinkhandover.

The method in a first radio base station comprising the steps ofreceiving information of the last packet (flag) to be transmitted to theUE from said first radio base station (Node B1) on a first connection,and informing the UE of the last packet to be transmitted to the UE fromsaid first radio base station (Node B1) on the first connection, makesit possible to achieve both lossless and continuous transmission at adownlink handover.

The method in a UE comprising the steps receiving a notification fromthe first radio base station (Node B1) that currently serves said UE,said notification indicating the last packet (flag) to be transmitted tothis user equipment from said first radio base station (Node B1) on afirst connection; and informing the second radio base station (Node B2)by a request to commence transmission from this second radio basestation (Node B2) when the notified last packet (flag) is received atthe UE, makes it possible to achieve both lossless and continuoustransmission at a downlink handover.

Moreover, the present invention is applicable also in cases where the UEis connected to a radio base station with several logical channels(“connections”). Therefore, the network node comprises means fordetermining a last packet to be transmitted on a second connection formthe first radio base station, and means for informing the UE of the lastpacket to be transmitted to the UE from said first radio base station onthe second connection and the UE comprises means for receivinginformation of the last packet to be transmitted to the UE from saidfirst radio base station on a second connection, and means for informingthe UE of the last packet to be transmitted to the UE from said firstradio base station on the second connection.

According to embodiments of the invention the determination of the lastpacket is based on measurement reports received from the UE or on thefill level of the buffer of the first radio base station.

Information of the last packet to be transmitted may be achieved byattaching a notification of the last packet to be transmitted to thelast packet to be transmitted or transmitting a notification of the lastpacket to be transmitted in a control packet or control signal.

According to a further embodiment, an execution time that is associatedwith the time for handover may be provided by the network. That meansthat the execution time is transmitted to the UE via the base stationand the handover is performed at the execution time if the executiontime elapses before the last packet to be transmitted is received at theUE.

The first radio base station according to one embodiment comprises meansfor prioritizing transmission of packets belonging to UEs that areassigned for handover.

An advantage with the present invention is that it allows forseamlessness and lossless handover. Seamlessness implies that the secondradio base station commence transmitting and scheduling as soon as thefirst radio base station has successfully finished its responsibilitiesfor that UE, and lossless implies that the first radio base station isallowed to continue its transmission until all packets in its buffersare successfully received in the UE.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a mobile cellular network wherein the present invention maybe implemented.

FIG. 2 shows a part of a cellular mobile communication system andillustrates the problem that the present invention intends to solve.

FIG. 3 also shows a part of a cellular mobile communication system andillustrates schematically the solution according to the presentinvention.

DESCRIPTION OF THE INVENTION

The present invention will be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein; rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art.

This application concerns any kind of mobile communication networkhaving demands on user-plane buffering in the radio base station anddemands on targets of supporting seamless and lossless downlinkhandovers from one radio base station to another.

The present invention is based on co-ordinating the timing of a downlinkcell change with the user-plane data transmission, so that thecell-change is executed directly upon successful reception of the lastdata available in an “old” radio base station node. Due to thisco-ordination it is possible to achieve fast, seamless and losslesshandovers.

The solution according to the present invention is explained below andin conjunction with FIG. 3. FIG. 3 illustrates the same network as FIG.2. When assuming that a downlink handover shall take place from a firstradio base station, e.g. Node B1, to a second radio base station, e.g.Node B2, an indication of the last packet to be transmitted from thefirst radio base station is introduced according to the presentinvention. In a first step, a controlling node, e.g. an RNC, sends anindication, referred to as a flag, of the last packet to be transmittedfrom the first radio base station, e.g. the Node B1 as illustrated inthe FIG. 3. Then, in a second step the controlling node notifies the UEof the last packet to be sent via the first radio base station, Node B1.Upon correct reception of that packet and the aforementioned indicationand preferably after having terminated all HARQ processes towards the“old” radio base station, the UE sends in a third step an uplinkrequest, typically on some contention-based channel, to the second radiobase station indicating that said second radio base station is now incharge of the UE and should commence transmitting as soon as possible. Acontention-based channel implies that collisions may occur, and anexample of such a contention-based channel is a random access channelwhere a transmitter awaits a random time for transmission if a collisionhas occurred. The above mentioned controlling node is a node adapted tocontrol one or more radio base stations. An example of such acontrolling node is a RNC in a WCDMA network.

Hence, the solution relies on means for informing the UE of the flagindicating the last packet to be transmitted from the “old” (i.e. thefirst) radio base station node. The indication must be based on asequence number known both in the “old” and the “new”, i.e. the second,radio base station node; and means for informing the “new” radio basestation node by means of a message from the UE to the “new” radio basestation. The message comprises a request to commence transmission fromthe “new” radio base station node. This message is sent from the UE whenthe afore-mentioned flag is received meanwhile (i.e. during, before, orafter the reception of the flag) the controlling node is performing anynecessary configurations to the “new” radio base station node and theUE. The necessary signalling procedures for this are specific for theapplied access technology. For WCDMA, examples of such configurationsequences can be found, e.g., in 3GPP TS 25.931.

Further, the present invention concerns means in the controlling nodefor deciding upon the last packet to be transmitted from the “old” radiobase station node, also referred to as the first base station. Thedecision can be based on information provided by the radio base stationnodes, including information on the buffer fill levels and downlink linkquality estimates provided by measurement reports transmitted from theUE. An embodiment of the present invention also concerns a scheduler inthe radio base station node that prioritizes the transmission of databelonging to UEs that are in progress of performing a handover. Inaddition, the Node B may comprise means for overriding the decision ofthe controlling node e.g. when the radio channel conditions are poorsuch that the packets determined by the controlling node cannot betransmitted.

When the controlling node decides to do a handover, the user-planeanchor point node, also referred to as the controlling node, attachesinformation of the last packet to be transmitted as a “flag” or similarnotification to the last packet to be transmitted from the “old” radiobase station node as described above. In a conceivable alternative tothis embodiment, e.g. if there are no packets to be transmitted to theUE in question, a specific control packet or control signal without anyuser-plane payload including the flag indicating the last packet thathas been forwarded to the “old” radio base station node, is sent to this“old” radio base station node. This indication can be sent ascontrol-information to the UE using any access specific methods andchannels for transmitting control information to the UE. This controlsignal, and the notification (“flag”) could then potentially be sent tothe radio base station node prior to the packet it references to.

The controlling node also defines a “handover execution time” which ittransmits to the UE via the old base station according to existing art.Thus the execution time is associated with the time for performing thehandover, and if the execution time elapses before the indicated lastpacket is received at the UE or transmitted from the network, thehandover is performed at said execution time. This works as an“over-ride” to ensure that the handover is executed also in cases whenthe “old” radio base station node fails to empty its buffer(s), i.e. itcannot successfully transmit its buffer up to the last packet residingin the buffer. The handover is then executed when a timing offset(derived according to existing art) elapses. According to oneembodiment, the “old” radio base station node Node B1 is notified ofthis execution time, so that the scheduler in the “old” radio basestation node Node B1 can optimize its scheduling decisions, e.g. byensuring that the relevant buffers are emptied on time.

Yet another embodiment of the present invention concerns a radio basestation scheduler that prioritizes the scheduling of UEs, which havebeen assigned for a handover, by prioritizing the remaining packets forthese UEs. This embodiment is closely related to the above mentionedembodiment. The benefits of such a scheduler include that the handoveris executed faster, and also that the “old” radio base station candisconnect these UEs as soon as possible, which is a benefit as theseUEs probably can consume a significant amount of resources due to theirlow link quality.

According to a further embodiment, the controlling node is assisted inthe timing of the handover decision by the involved radio base stations,so that the radio base stations provide information of the presentbuffer fill-levels and link quality estimates. This information servesas an estimate of the handover execution time and the resourceconsumption for transmitting the last few packets from the “old” radiobase station. In cases when the “old” radio base station has either verylarge buffer(s) or a low link quality to the UE in question, or both,the controlling node may decide to flag for the handover to be executedbased on a packet which is not the last one forwarded to the “old” radiobase station. I.e. there are remaining packets in the buffer of the“old” radio base station when the handover is performed. In such case,to achieve lossless handover, the packets that will not be forwardedfrom the “old” radio base station to the UE has to be re-routed orbi-casted to the “new” radio base station.

Moreover, it should be noted that the present invention is applicablealso in cases where the UE is connected to a radio base station withseveral logical channels (“connections”). In such cases, multiplepackets on separate logical channels may be indicated as the last packetto be transmitted, and the activation of the handover is performed basedon some logical operation saying, e.g., that the handover is activatedwhen all the referenced packets have been successfully received.Alternatively, the last packet to be transmitted may indicate a packeton a single logical channel, and the handover is executed when thispacket has been arrived, irrespectively of the situation on otherlogical channels.

The present invention is not limited to the above-described preferredembodiments. Various alternatives, modifications and equivalents may beused. Therefore, the above embodiments should not be taken as limitingthe scope of the invention, which is defined by the appending claims.

The invention claimed is:
 1. A method in a network node of a cellularmobile communication network for handover of a user equipment, UE, froma first radio base station to a second radio base station, comprisingthe steps of: determining a last packet to be transmitted to the UE fromthe first radio base station on a first connection, determining a lastpacket to be transmitted to the UE from the first radio base station ona second connection; informing the UE of the last packet to betransmitted to the UE from said first radio base station on said firstand second connections; transmitting an execution time that isassociated with a latest time for performing handover, such thathandover is performed when the last packet is received at the UE on atleast one of said first and second connections if the last packet isreceived before the execution time elapses, and the handover isperformed at the execution time if the execution time elapses before thelast packet to be transmitted on at least one of said first and secondconnections from the first radio base station is received at the UE. 2.The method according to claim 1, wherein the UE is informed via thefirst radio base station that currently serves said UE.
 3. The methodaccording to claim 2, wherein the determination of the last packet isbased on the fill level of the buffer of the first radio base station.4. The method according to claim 1, wherein the determination of thelast packet is based on measurement reports received from the UE.
 5. Themethod according to claim 1, wherein the informing step comprises thestep of: attaching a notification of the last packet to be transmittedto the last packet to be transmitted.
 6. The method according to claim1, wherein the informing step comprises the step of: transmitting anotification of the last packet to be transmitted in a control packet orcontrol signal.
 7. A method in a first radio base station of a cellularmobile communication network for handover of a user equipment, UE, fromthe first radio base station to a second radio base station, comprisingthe steps of: receiving information of a last packet to be transmittedto the UE from said first radio base station on a first connection,receiving information of a last packet to be transmitted to the UE fromsaid first radio base station on a second connection, informing the UEof the last packet to be transmitted to the UE from said first radiobase station on the first and second connections; receiving an executiontime that is associated with a latest time for performing handover, suchthat handover is performed when the last packet is received on at leastone of said first and second connections at the UE if the last packet isreceived before the execution time elapses, and the handover isperformed at the execution time if the execution time elapses before thelast packet is received on at least one of said first and secondconnections at the UE; and transmitting said execution time that isassociated with the latest time for performing handover to the UE. 8.The method according to claim 7, wherein the information comprises anattached notification to the last packet to be transmitted.
 9. Themethod according to claim 7, wherein the information is transmitted bymeans of a control packet or a control signal.
 10. The method accordingto claim 7, wherein it comprises the further step of: scheduling packetsto be transmitted in order to empty the buffer of the first radio basestation within the received execution time.
 11. The method according tothe previous claim 10, wherein it comprises the further step of:prioritizing transmission of packets belonging to UEs that are assignedfor handover.
 12. A method in a user equipment, UE, of a cellular mobilecommunication system for a handover from a first radio base station to asecond radio base station, comprising the steps: receiving anotification from the first radio base station that currently servessaid UE, said notification indicating a last packet to be transmitted tothis user equipment from said first radio base station on a firstconnection; receiving a notification from the first radio base stationthat currently serves said UE, said notification indicating a lastpacket to be transmitted to the user equipment from said first radiobase station on a second connection; receiving an execution time that isassociated with the latest time for performing handover; performinghandover at the execution time if the execution time elapses before thelast packet is received on at least one of said first and secondconnections; performing handover when the last packet is received if thelast packet is received on at least one of said first and secondconnections before the execution time elapses, and informing the secondradio base station of execution of handover by a request to commencetransmission from this second radio base station when the last packet onat least one of said first and second connections is received.
 13. Themethod according to claim 12, wherein it comprises the further step of:terminating the HARQ-processes towards the first radio base station andcollecting re-configuration data.
 14. A network node of a cellularmobile communication network adapted to control handover of a userequipment, UE, from a first radio base station to a second radio basestation, comprising: means for determining a last packet to betransmitted to the UE from the first radio base station on a firstconnection; means for determining a last packet to be transmitted to theUE from the first radio base station on a second connection; means forinforming the UE of the last packet to be transmitted to the UE fromsaid first radio base station on said first and second connections;means for transmitting an execution time that is associated with alatest time for performing handover, such that the handover is performedwhen the last packet is received at the UE on at least one of said firstand second connections if the last packet to be transmitted from thefirst radio base station is received before the execution time elapses,and handover is performed at the execution time if the execution timeelapses before the last packet to be transmitted on at least one of saidfirst and second connections from the first base station is received atthe UE.
 15. The network node according to claim 14, wherein the UE isinformed via the first radio base station that currently serves said UE.16. The network node according to claim 15 wherein the determination ofthe last packet is based on the fill level of the buffer of the firstradio base station.
 17. The network node according to claim 14, whereinthe determination of the last packet is based on measurement reportsreceived from the UE.
 18. The network node according to claim 14,wherein the means for informing comprises means for attaching anotification of the last packet to be transmitted to the last packet tobe transmitted.
 19. The network node according to claim 14, wherein themeans for informing comprises means for transmitting a notification ofthe last packet to be transmitted in a control packet or control signal.20. A first radio base station of a cellular mobile communicationnetwork adapted for handling handover of a user equipment, UE, from thefirst radio base station to a second radio base station, comprising:means for receiving information of a last packet to be transmitted tothe UE from said first radio base station on a first connection, meansfor receiving information of a last packet to be transmitted to the UEfrom said first radio base station on a second connection: means forinforming the UE of the last packet to be transmitted to the UE fromsaid first radio base station on the first and second connections; meansfor receiving an execution time that is associated with a latest timefor performing handover, such that the handover is performed when thelast packet is received at the UE on at least one of said first andsecond connections if the last packet is received before the executiontime elapses, and handover is performed at the execution time if theexecution time elapses before the last packet to be transmitted on atleast one of said first and second connections from the first basestation is received at the UE; and means for transmitting said executiontime that is associated with the latest for performing handover to theUE.
 21. The first radio base station according to claim 20, wherein theinformation comprises an attached notification of the last packet to betransmitted.
 22. The first radio base station according to claim 20,wherein the information is transmitted by means of a control packet or acontrol signal.
 23. The first radio base station according to claim 20,wherein it comprises the further means for scheduling packets to betransmitted in order to empty the buffer of the first radio base stationwithin the received execution time.
 24. The first radio base stationaccording to the previous claim 23, wherein it comprises the furthermeans for prioritizing transmission of packets belonging to UEs that areassigned for handover.
 25. A user equipment, UE, of a cellular mobilecommunication system adapted to perform handover from a first radio basestation to a second radio base station, comprising: means for receivinga notification from the first radio base station that currently servessaid UE, said notification indicating a last packet to be transmitted tothis user equipment from said first radio base station on a firstconnection; means for receiving a notification from the first radio basestation that currently serves said UE, said notification indicating alast packet to be transmitted to the user equipment from said firstradio base station on a second connection; means for receiving anexecution time that is associated with a latest time for performinghandover, wherein the UE is adapted to perform handover at the executiontime if the execution time elapses before the last packet is received onat least one of said first and second connections, and the UE is adaptedto perform handover when the last packet is received if the last packetis received on at least one of said first and second connections beforethe execution time elapses; and means for informing the second radiobase station of execution of handover by a request to commencetransmission from the second radio base station when the last packet onat least one of said first and second connections is received.
 26. TheUE according to claim 25, wherein it further comprises means forterminating the HARQ-processes towards the first radio base station andcollecting re-configuration data.